During intracellular vesicular trafficking, delivery of Rab guanosine triphosphatases (GTPases) to sites of activation involves their release from a guanine nucleotide dissociation inhibitor (GDI) by a so-called GDI displacement factor (GDF). The identification of GDFs has been difficult. Now Machner and Isberg provide evidence for the existence of such an activity, surprisingly in a bifunctional protein--a guanine nucleotide exchange protein associated with the formation of a replication vacuole by the intracellular pathogen Legionella pneumophila.